Mercury (Hg) is a widely disseminated toxicant found in various occupational, environmental and domestic settings. It exerts a wide range of cellular toxicities in various human tissues and organs. However, at concentrations that do not induce acute cellular death, mercury induces a variety of genes (in target cells), including those encoding metallothioneins (MTs), presumably through the action of metal-responsive transcription factors. Recently, particular attention has been paid to the effects of mercury and other metals on the physiology of mammalian leukocytes. Activation is one of the vital physiological responses that appear to be influenced by mercury and other metals. Activation is the acute phase response and respiratory burst undergone by monocytes (and other leukocytes) as a response to extracellular events and signals. As such, activation involves a series of events regulated by signals that must be appropriately received, transduced to the nucleus, and processed to trigger transcription and/or repression of genes. We have shown that treatment of human monocytes with mercury (and other metals), at levels lower than the minimum required to exert acute cellular injury or death, significantly inhibits the ability of bacterial lipopolysaccharide (LPS) to trigger activation. Acute and/or chronic suppression of monocyte-activation by mercury may prove to have significant deleterious health effects in humans. A principal aim of this proposal is to evaluate, in vitro and in vivo, the inhibitory effect of very low concentrations (pM-pM) of inorganic mercury on: a) LPS-, cytokine-, and phorbol ester-induced activation of primary (mouse and human) and immortalized (human) monocytes (by assessment of multiple cellular and molecular measures of the activation process) and induced differentiation into macrophages. Since MTs are both induced by metals and have been proposed as intracellular regulators of the availability of essential metals, and play an important role in oxygen radical scavenging and monocyte activation, we will assess the role of MTs in mercury-induced inhibition of activation. To test the hypothesis that MTs play a role in mercury-mediated suppression of activation, we will study the activity of transcription factors and signal transduction proteins (protein kinase C, 1K-B, and NF-KB), in: a) primary monocytes from MT "knockout" mice, and transgenic (MT about1*) mice that over express MT-i, are treated or untreated with inorganic mercury, before and after activation; and b) human monocytes expressing different MT-isoforms under the control of an IPTG-inducible promoter, before and after activation. Low, non-toxic levels of Hg have significant effects on the capacity of monocytes to respond to activation signals. We will fully characterize the effects of Hg on primary monocytes and a monocyte cell line, and test the hypothesis that MTs induced by mercury alter the activation-process by modulating the activity of proteins critical in mediating those signals.